1. Field of the Invention
The present invention relates to a fault deciding system for detecting a fault of an idle revolution controlling unit of an engine.
2. Related Art
An idle RPM (revolution-per-minute) of the engine is controlled by adjusting the degree of opening of an EACV (Electronic Air Control Valve) bypassing a throttle valve when the throttle valve is fully closed, to control the air flow to be fed to the engine. In this case, the idle RPM controlling unit is determined to be normal when the actual idle RPM Ne detected by the RPM controlling unit integrated circuit (IC) falls within the following normal region: EQU Target RPM-100 rpm.ltoreq.Ne.ltoreq.Target RPM+200 rpm.
The idle RPM controlling unit IC is determined to be faulty when the actual idle RPM is not within the previously specified range.
The actual idle RPM Ne is detected as the engine RPM for the fully closed throttle opening value, which in turn is detected in terms of the output voltage value of a throttle opening sensor. The throttle opening value is decided to be fully closed when the output value TH of the throttle opening sensor falls within the region which is defined by a preset fully closed value TH.sub.0 and a fixed value .alpha., as follows: EQU TH.sub.0 .ltoreq.TH.ltoreq.TH.sub.0 +.alpha..
If, however, the fully closed throttle opening value is decided on the basis of the aforementioned preset fully closed value TH.sub.0, fault detection is impossible when the output value TH at the fully closed time of the throttle valve is raised to be greater than TH.sub.0 +.alpha. due to the aging of the throttle opening sensor.